1. Field of the Invention
This invention relates to the production of particles having well defined particulate shapes and sizes. More particularly, it relates to the production of nonspherical particles such as ellipsoids and spheroids, preferably monodisperse in particle dimension.
2. Description of the Prior Art
The production of spherical particles in polymer latices is well known in the art. For example, the production of monodisperse spherical polystyrene particles by emulsion polymerization was first reported by Bradford and Vanderhoff, J. Appl. Phys., 26, 864 (1955) and Vanderhoff et al., J. Polym. Sci., 20, 255 (1956).
A comprehensive review of the preparation and use of monodisperse polymer particles was reported by Hearn et al. in Adv. Colloid Interf. Sci., 14, 173 (1981). Although monomers such as vinyl chloride, butadiene, methylmethacrylate, vinyl acetate, and isoprene have been used for preparation of polymer latices, the majority of research and development has been done with styrene.
Emulsion polymerization can be carried out in the presence or absence of added surfactants. Polymerization initiators may be free radicals produced by dissociation of peroxy and azo compounds. Alternatively, a redox system such as a persulfate/sulfite system may be employed to initiate emulsion polymerization.
Polymer particles that are spherical in shape and exceptionally monodisperse in size can be prepared by any of the numerous emulsion polymerization techniques. The diameters of the spherical microparticles produced by emulsion polymerization methods can range from 0.01 to greater than 1.0 microns. The chemical composition and surface properties of such spherical particles can be varied over a wide range.
Seeded emulsion polymerization techniques can be used to produce microspheres having larger diameters. Ugelstad et al. Makromol, Chem., 180, 737 (1979); Ugelstad et al. Ady. Colloid Interf. Sci., 13, 101 (1980). Particles with diameters up to 20 microns can be easily produced by several successive seeded growth steps.
Monodisperse metal sols have been produced by techniques of homogeneous precipitation and have been prepared from a variety of elements, including gold, sulfur, selenium, and silver. Zsigmondy, Z. Physik. Chem., 56, 56 (1906); Zsigmondy, Z. Anorg. Chem., 99, 105 (1917); Zsigmondy & Thiessen, Das Kalloide Gold, Akad. Verlagsges, Liepzig (1925); Lamer & Barnes, J. Colloid Sci., 1, 71 (1946); and Ottewill and Woodbridge, J. Colloid Sci., 16, 581 (1961).
The need for well-characterized nonspherical particles, however, remains. Nonspherical particles produced to date include the following: inorganic sols of metal hydrous oxides, Matijevic et al., Prog. Colloid Polym. Sci., 61, 24 (1976), Pure Appl. Chem., 50, 1193 (1978), Acc. Chem. Res., 14, 22 (1981), Demchak & Matijevic, J. Colloid Interf. Sci., 31, 257 (1969), and Brace and Matijevic, J. Inorg. Nucl. Chem., 35, 3691 (1973); spindle-shaped silica particles, Ogino and Kuronoma, J. Colloid Interf. Sci., 56, 629 (1976); and frozen erythrocytes, Sutera and Mehrjardi, Biophys. J., 15, 1 (1975). Most of these particles, however, are quite polydisperse, having complex nonspherical shapes, rough surfaces, and significant particle size variability. Little work has been done to systematically vary their surface properties. More importantly, the size and shape of particles cannot be systematically controlled and varied over a wide range.
It is, therefore an object of the present invention to produce particles having simple geometric shapes other than a sphere.
Another object of the present invention is to produce simple particles that are monodisperse both in size and shape.
It is still another object of this invention to produce microparticles having uniform prolate spheroidal, oblate spheroidal, and ellipsoidal shapes.
It is a further object of this invention to provide a process for systematically controlling microparticle size, shape, and surface properties.